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I am experiencing high idle (around 12-1300 RPM), stumbling at speed, and although I do have a good bit of torque it still feels like it's down on power.

I datalogged a recent drive, and have the following observations:

-TPS Volt is way out of range- 0.14V at idle, and IIRC about .4 volts at heavy throttle

-IAC counts may be out of range as well- I'm still learning this stuff

-Temp seems fine, at 194deg at speed and 199 at idle (ambient temp was about 75)

-Vacuum is ranging from about 19 at idle, to an indicated -6 on heavy throttle. I believe I saw somewhere that a negative indication before the integer actually indicates positive pressure.

-I messed around under the hood looking for a vacuum leak, finding none. I did find the vacuum reservoir had a hose that had dropped onto a manifold and melted in two. Can not find where that is supposed to go, however. I haven't found any vacuum hoses "adrift" in the engine bay. Only thing that has a loose hose is the cruise control servo and the trans vent. This bears further investigation, that reservoir used to go somewhere, I need to find it. However, given that I have 19 inches of vac at idle I'm thinking it can't be much of a leak.

-While chasing down the above with engine off, I had my hands pretty much on everything. Afterwards, I cranked and it was idling at about 800RPM. I applied the throttle and let go, and idle would not go below 1100. I almost wonder if this is indicative of a throttle cable with too much stiction- it's 30 years old, maybe it just needs to be replaced like the TV cable on the trans was?

If this is a transplant,triple check your grounds. But looking at the TPS voltage, it's definitely got issues. Clear DTC's, unplug the TPS, turn key off, push pedal from closed to full throttle and back to closed. wait 3 seconds turn key to run position then recheck TPS voltage. If TPS is still out of whack, get a volt meter set to DC volts and check grounds between the TPS, engine block, battery, and body. TPS will have 3 wires. 5volt DC reference voltage. Ground. and Signal return. If all these items check out, replace the TPS.

I think if you look at the wiring diagram, you should see that those sensors share a ground that, on the full size C/K tucks is attached to a stud on the thermostat housing. Are you using the v6 or v8 harness? Also, how is your MAP sensor attached vacuum-wise? Should not be any troughs or loops if at all possible and closer to the port(shortest vacuum hose possible) is best, usually at the center, rear of the throttle body.

All of my grounds are via 4 gauge wire, soldered to ring terminals, and go from battery to frame, battery to block, block to block (at each side of the intake manifold, as well as from driver's front to back of the intake, and from driver's back of manifold to the body as well as driver's block forward side of manifold to frame. (Yes, I'm kinda ground-paranoid).

That said, I abandoned the thermostat housing stud ground as that's always irritated me when it comes time to service the cooling system. I can't imagine how the aluminum intake would be not grounded, what with 3 of the bolts holding it down having grounds attached with star washers, but I will check just to make sure and maybe run a ground wire to the boss that formerly held the coil (although in all honesty, it's already explicitly grounded by the strap at the throttle bracket, but ok). Not being grounded at that exact point the factory used doesn't matter, especially as I've massively upgraded the conductivity of the grounds.

Don't know what "clear DTC" means.... will GTS.. :)

This is a transplant into an 89 S10 Blazer. It originally had the 4.3 and auto trans. The TBI harness is compatible between the two engines. I am using the computer that came with the engine, it was a plug-and-play operation.

One thing to note- I don't have the "idle-up on crank" that I had with the 4.3. As they use the same TBI arrangement I would expect that to be applicable here as well. Some quick research tells me this points to the IAC.

MAP sensor is attached to the factory port on the front of the TBI, I replaced the fragile and broken hard vac line with similarly (ID) sized fuel hose (I always do this as it's much more durable and I hate chasing vac leaks). It's the minimum length necessary to enable nice bend radius , and is within 20% +/- of the factory hard hose length.

Now that you mention the center rear of the TBI, I hav to admit I may have an open port there. But how on earth could I have that and still have 19" of vac at idle?

DTC = Diagnostic Trouble Code Check engine light is on. Clear the codes by removing battery terminal for 30 seconds or so. I agree with buddrow. Check the grounds and connections for the sensor grounds. Sensor ground from the ecm has to have a good electrical connection to the engine block and battery ground. Map sensor should be connected to the port on the back of the TBI. See the first post in this thread: http://www.gearhead-efi.com/Fuel-Inj...n-vacuum-ports

Replaced the TPS as a matter of course once I realized how out of spec it was as well as non-adjustable. New unit shows approx 0.8 volts at rest, 5 at WOT. I think this is near perfect.

Cleared codes, went for a long drive. No CEL, though I do still have some drivability tweaking to do.

I think the "-3" and such Vacuum figures may be due to a bad head gasket as a friend told me. I'll investigate that and proceed from there. I'll test for combustion products in the coolant, and if I don't see evidence of issues in the oil or water, I will run a head gasket sealant first since the issue won't be extreme. Retest and assess.

Now it's not so late at night, I will post in more detail-
I checked my grounds- everything is ringing back to the negative terminal of the battery at around 0.3-0.9 ohms, So we'll call that good.

Removed the TPS, finding it's non-adjustable. I may have the range the new one operates in misremembered. Will dig in and get better data soon.

I swapped the MAP and EGR vac feeds- MAP now feeds from the large port at front of the TBI. EGR feeds from the port at the back of the TBI. I do need to find out where the vacuum reservoir is intended to attach. I am running hydroboost braking so the runner out of the plenum rear of the TBI is capped off with a rubber cap for now. I will install a proper pipe plug soonest. I have never had a vacuum line to the driver's side valve cover, so the driver's side TBI unit vacuum port is capped off. I have a vacuum line going from the TBI unit to the vapor cannister, and that's all I can remember at present. I have a grommet in the passenger's side with a PCV in it, hose going to the intake runner IIRC. There was a line going from the driver's side valve cover between #5 and 7 to the spacer ring on the TBI unit, but I capped off the port in that ring and replaced the line with a breather in the valve cover. It's the "same difference" I suppose, with the exception that the breather isn't recirc'd into the intake. I've seen this done a lot on smallblocks and several smallblock expert friends advised it.

I cleared the codes by way of disconnecting the battery and then leaving the truck sit for an hour as I went to pick my son up from a function. Came back an hour later and reconnected. Yes this is way overkill and not needed, but it worked as I was able to be away and get something done as well.

Reconnected, and cranked the truck. I logged about 10 minutes of idling with occasional throttle blips in my driveway, watching the truck go into closed loop mode. During this, there was one instance of rough idle and dieing, as I fought a few months ago. I thought it might be a problem, but on refire it just ran fine. Pretty sure this was just part of the learning program for the ECU.

Went for a long (odo doesn't work so I am not sure how long, exactly- would estimate 40 miles or so- drive, on the same route as a few nights ago with lots of tooling around town as well. I have no CEL, which is honestly surprising as I thought I'd at least have an EGR error. So far, so good.

I do have a little bit of lugging when the trans shifts into overdrive, but I suspect this is more an adjustment needed in the transmission, than an engine issue.

Also, my brand new oil pressure sending unit is not showing much pressure on the dash. I tested the engine when I installed it, had great pressure.

The MAP sensor can be mounted literally anywhere as long as it has an unbroken hose attached between it and a manifold vacuum source. If you really wanted you could mount it on the tail light and it would still work fine. The only problem with a long hose is that the change in vacuum seems to get delayed more the longer the hose is. So an engine mounted MAP sensor, even if the hose is an extra 6 or 8 inches over what the original hose was is going to work just fine. No need to chase ghosts.

The man who says something is impossible, is usually interrupted by the man doing it.

Pretty sure he's referring to the location of the vac source, not the physical location of the sensor itself.

That said, I am going to move the MAP to the firewall, still with the same source, simply as a way to get rid of that atrocious bracket for the MAP and EGR Solenoid. I'll ziptie the EGR solenoid to the harness until I am able to do an EGR-delete PROM mod, at which time I'll simply remove that sensor entirely.

I've tested the alternator since I'm seeing what appears to be the beginning of the end with it, and I got indeterminate results- tested 3 separate times, I got 2 passes and 1 fail. That, coupled with a borderline indication at hot idle or hot run, tells me it's time it's out of there.. and since I am putting winches and multiple batteries on this truck, it's time to upgrade. I'll be going with an AD244 alternator from a 2002 Tahoe, which will bolt right in (though I will need to change the connector). I think this may be a small part of my issue, as I noticed this afternoon while doing a check ride there was a stumble while the alternator dipped out once or twice.

Pretty sure he's referring to the location of the vac source, not the physical location of the sensor itself.

That might be, but the way it's written doesn't say that. It's getting more and more difficult to understand what people are writing lately due to a lack of complete thoughts being written anymore. Forums are usually not bad, but social media... Oh man, just such lazy writing on there...

But I digress...

[/quote]That said, I am going to move the MAP to the firewall, still with the same source, simply as a way to get rid of that atrocious bracket for the MAP and EGR Solenoid. I'll ziptie the EGR solenoid to the harness until I am able to do an EGR-delete PROM mod, at which time I'll simply remove that sensor entirely.

I've tested the alternator since I'm seeing what appears to be the beginning of the end with it, and I got indeterminate results- tested 3 separate times, I got 2 passes and 1 fail. That, coupled with a borderline indication at hot idle or hot run, tells me it's time it's out of there.. and since I am putting winches and multiple batteries on this truck, it's time to upgrade. I'll be going with an AD244 alternator from a 2002 Tahoe, which will bolt right in (though I will need to change the connector). I think this may be a small part of my issue, as I noticed this afternoon while doing a check ride there was a stumble while the alternator dipped out once or twice.[/QUOTE]

Yeah, it's sometimes difficult to make the TBI motors look good, at least without moving a lot of stuff. That bracket is just horrible, but they were in trucks, not show cars, so they didn't need to look good, just practical. lol

Last edited by Six_Shooter; 09-11-2016 at 08:32 PM.

The man who says something is impossible, is usually interrupted by the man doing it.